1. Field of the Invention
This invention relates to a wire bonding device and in more particular to a wire bonding device which performs digital control of a bonding head.
2. Description of the Prior Art
As shown in FIG. 1, a prior wire bonding device is comprised of one television camera 1, a position detector 3, and a bonding head controller 6.
An image of the semiconductor pellet 7 is picked up using the television camera 1, and the video signal is input to the position detector 3. In the position detector 3, the positioning data for the semiconductor pellet 7 is calculated based on the input video signal, and the data is input to the bonding head controller 6. In the bonding head controller 6, the position of the bonding tool (not shown in the figure) is controlled based on the input positioning data for the semiconductor pellet 7, and metal wiring (not shown in the figure) is bonded to the semiconductor pellet 7. The variation over time of the end position of the bonding tool is shown in FIG. 2A. First, the bonding tool stops at an initial position h.sub.0, and when the bonding operation begins it is rapidly lowered with time. Immediately after it reaches the height h.sub.1, the lowering speed is decreased and it continues to be lowered toward the pellet position h.sub.2. As soon as the bonding tool reaches the pellet position h.sub.2 it stops and after a short wait it is raised in the direction of the initial height h.sub.0.
The height h.sub.1 where the speed changes during lowering of the bonding tool is called the search height. The speed is changed at this height because if the bonding tool strikes the semiconductor pellet 7 at high speed, cracks may occur in the semiconductor pellet 7 due to the impact, and so it is necessary to decrease the lowering speed of the bonding tool.
The pellet position h.sub.2 is detected by a touch sensor (not shown in the figure) which is assembled in the bonding tool.
In the prior wire bonding device as mentioned above, an image of the semiconductor pellet is picked up using one television camera and the position of the semiconductor pellet is processed in the position detector using two-dimensional coordinates. The pellet position is detected using the touch sensor, and therefore, if the bonding operation is not actually performed, the height of the semiconductor pellet is unknown. If the height of the semiconductor pellet is changed from h.sub.2 to h.sub.20 as shown in FIG. 2B, the search height still remains fixed and so there are variations in the speed at which the bonding tool strikes the surface of the semiconductor pellet. This causes deformation in the end shape of the metal wiring (crushed too or short) and lowers the product quality, which is a problem.